Thermally foamable microspheres, also called thermally expansible microcapsules, are now put into practical use in various fields, for instance, in the form of fillers for paints or plastics added with a view to weight reduction purposes, to say nothing of foaming inks.
Thermally foamable microspheres have usually volatile liquid foaming agents microcapsulated with polymers. Such foaming agents are also called physical or volatile foaming agents. If desired, a chemical foaming agent that is decomposed upon heating to generate gases may often be used.
Generally, the thermally foamable microspheres may be produced by the suspension polymerization in an aqueous medium of a polymerizable mixture that at least contains a foaming agent and a polymerizable monomer. As the polymerization reaction goes on, a shell is formed by the resulting polymer, yielding a thermally foamable microsphere having a structure wherein the foaming agent is wrapped up and encapsulated in the shell.
For the polymer that forms the shell, thermoplastic resins having satisfactory gas barrier properties are generally used. The shell-forming polymer softens upon heated. The foaming agents used are selected from those that gasify at temperatures lower than the softening point of the polymer.
As the thermally foamable microsphere is heated, the foaming agent is vaporized to generate expanding force acting on the shell. At the same time, however, there is a drastic drop of the modulus of elasticity of the shell-forming polymer. For this reason, drastic expansion occurs at a certain critical temperature that is referred to as the foaming temperature. In other words, the thermally foamable microsphere expands by itself upon heated to the foaming temperature, forming a closed cell (cellular particle).
Taking advantage of the ability to form closed cells, thermally foamable microspheres have currently found a wide range of applications in the form of aesthetic improvers, functionality givers, weight reducers, etc. As the performance demanded for each application grows, the level demanded for the thermally foamable microspheres grows, too. Among the properties demanded for the thermally foamable microspheres, there is an improvement in processability.
For instance, there is a method for obtaining formed articles or sheets whose weight is reduced or whose aesthetic properties are improved by subjecting a composition comprising a thermoplastic resin blended with thermally foamable microspheres to kneading, calendering, extrusion or injection molding during which the thermally foamable microspheres are foamed.
As the thermally foamable microsphere undergoes volume expansion during foaming, however, the shell-forming polymer layer becomes thinner and thinner. In addition, the modulus of elasticity of the shell-forming polymer drops drastically under the influences of high processing temperature and high shear force with the result that the shell becomes soft and so the thermally foamable microsphere breaks down easily. It is thus very difficult to achieve the desired object.
Another problem with the thermally foamable microsphere is that the proper processing temperature range is very narrow because of an increased temperature dependency of the modulus of elasticity of the polymer that forms the shell.
Further, conventional thermally foamable microspheres are poor in resistance to polar solvents and plasticizers (solvent resistance and chemical resistance), and so have only limited applications in fields where, for instance, polar organic solvents are used.
JP-A 11-60868 discloses a soft vinyl chloride resin composition for foaming extrusion molding, in which thermally expansible microcapsules are blended with a plasticizer-containing vinyl chloride resin.
JP-A 2000-17103 discloses a resin composition production process comprising the first step of kneading at 100° C. or lower a resin composition comprising a thermoplastic resin having a melting or softening point of 100° C. or lower and a thermally expansible microcapsule that expands at 100 to 200° C., and the second step of kneading or molding the resultant resin composition with the addition of a thermoplastic resin thereto.
In order that thermally foamable microspheres are actually applicable to such foaming extrusion molding or kneading/molding, they must have a shell that has high foaming temperature and improved heat resistance. To add to this, the modulus of elasticity of the polymer that forms the shell must have a decreased dependency on temperature, a wide range of proper processing temperature, and improved resistance to polar solvents, plasticizers, etc.
To produce thermally foamable microspheres having high heat resistance, a process for forming a shell-forming polymer layer by the polymerization of a polymerizable monomer comprising a vinyl monomer with the addition of a crosslinkable monomer thereto has already been put forward (see JP-B's 42-26524 and 5-15499, U.S. Pat. No. 2,894,990 and JP-A 5-285376). By use of the crosslinkable monomer, it is thus possible to introduce a crosslinked structure into the shell-forming polymer, thereby making improvements in the heat resistance and melt flowability of thermally foamable microspheres.
As the degree of crosslinking of the polymer that forms the shell becomes high, however, the thermal expansibility of thermally foamable microspheres is impaired. For this reason, each of the examples in these prior arts merely shows that the crosslinking agent is used at a very low proportion of 1% by weight or less, and preferably 0.2 to 0.6% by weight of the polymerizable monomer.
At such a low proportion of the crosslinking agent it is impossible to obtain thermally foamable microspheres having sufficiently improved processability. Shells formed of conventional crosslinked polymers, too, are found to have a large dependency of their modulus of elasticity on temperature and, hence, have a very narrow range of proper processing temperature or be inferior in processability. Further, the shells formed of conventional crosslinked polymers are insufficient in terms of resistance to polar solvents and plasticizers.
Furthermore, the shells formed of conventional crosslinked polymers are actually limited to a polymer having a specific composition, and so it is difficult to design thermally foamable microspheres having improved compatibility with the thermoplastic resin used.